Method and system for asynchronous successive approximation register (sar) analog-to-digital converters (adcs)

ABSTRACT

An asynchronous successive approximation register analog-to-digital converter (SAR ADC), which utilizes one or more overlapping redundant bits in each digital-to-analog converter (DAC) code word, is operable to generate an indication signal that indicates completion of each comparison step and indicates that an output decision for each comparison step is valid. A timer may be initiated based on the generated indication signal. A timeout signal may be generated that preempts the indication signal and forces a preemptive decision, where the preemptive decision sets one or more remaining bits up to, but not including, the one or more overlapping redundant bits in a corresponding digital-to-analog converter code word for a current comparison step to a particular value. For example, the one or more remaining bits may be set to a value that is derived from a value of a bit that was determined in an immediately preceding decision.

CROSS-REFERENCE TO RELATED APPLICATIONS/INCORPORATION BY REFERENCE

This patent application makes reference to, claims priority to andclaims benefit from the U.S. Provisional Patent Application Ser. No.61/681,828, filed on Aug. 10, 2012.

This application also makes reference to:

U.S. application Ser. No. 13/945,579 Jul. 18, 2013, which claimspriority toU.S. Application Ser. No. 61/673,219, which was filed on Jul. 18, 2012;andU.S. Application Ser. No. 61/673,219, which was filed on Jul. 18, 2012.

The above stated application is hereby incorporated herein by referencein its entirety.

FIELD OF THE INVENTION

Certain embodiments of the invention relate to analog-to-digitalcircuits. More specifically, certain embodiments of the invention relateto a method and system for asynchronous successive approximationregister (SAR) analog-to-digital converters (ADCs).

BACKGROUND OF THE INVENTION

Successive approximation register (SAR) analog-to-digital converters(ADCs) may achieve high resolution but are limited in the speed they canachieve. Two components that limit the speed of asynchronous successiveapproximation register analog-to-digital converters are (1) the settlingtime of a digital-to-analog converter (DAC) that is utilized by the ADC,and (2) the metastability of the comparator that is utilized by the ADC.

Further limitations and disadvantages of conventional and traditionalapproaches will become apparent to one of skill in the art, throughcomparison of such systems with some aspects of the present invention asset forth in the remainder of the present application with reference tothe drawings.

BRIEF SUMMARY OF THE INVENTION

A system and/or method is provided for asynchronous successiveapproximation register analog-to-digital converters, substantially asshown in and/or described in connection with at least one of thefigures, as set forth more completely in the claims.

These and other advantages, aspects and novel features of the presentinvention, as well as details of an illustrated embodiment thereof, willbe more fully understood from the following description and drawings.

BRIEF DESCRIPTION OF SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 is a block diagram illustrating a successive approximationregister ADC, in connection with various exemplary aspects of theinvention.

FIG. 2 is a block diagram illustrating operation of an asynchronoussuccessive approximation register ADC, in connection with variousexemplary aspects of the invention.

FIG. 3 is a diagram that illustrates DAC redundancy and its effect onDAC settling time for bit n, tDS_n, in connection with various exemplaryaspects of the invention.

FIG. 4 is a block diagram of an exemplary asynchronous successiveapproximation register ADC, in accordance with an exemplary embodimentof the invention.

FIG. 5 is a diagram illustrating exemplary operation of an asynchronoussuccessive approximation register ADC, in accordance with an exemplaryembodiment of the invention.

FIG. 6 is a diagram illustrating an exemplary preemptive decision withredundancy for an asynchronous successive approximation register ADC, inaccordance with an exemplary embodiment of the invention.

FIG. 7 is a flow chart illustrating an exemplary preemptive decisionwithout timeout, in accordance with an exemplary embodiment of theinvention.

FIG. 8 is a flow chart illustrating an exemplary preemptive decisionwith timeout, in accordance with an exemplary embodiment of theinvention.

DETAILED DESCRIPTION OF THE INVENTION

Certain embodiments of the invention may be found in a method and systemfor asynchronous successive approximation register (SAR)analog-to-digital converters (ADCs). In various exemplary embodiments ofthe invention, the asynchronous SAR ADC, which utilizes one or moreoverlapping redundant bits in each digital-to-analog converter (DAC)code word, is operable to generate an indication signal that indicatescompletion of each comparison step and indicates that an output decisionfor each comparison step is valid. A timer in the asynchronous SAR ADCmay be initiated based on the generated indication signal. A timeoutsignal may be generated that preempts the indication signal and forces apreemptive decision. The preemptive decision causes one or moreremaining bits up to, but not including, the one or more overlappingredundant bits in a corresponding digital-to-analog converter code wordfor a current comparison step to be set to a particular value. In anexemplary embodiment of the invention, the one or more remaining bits upto, but not including, the one or more overlapping redundant bits in thecorresponding digital-to-analog converter code word for the currentcomparison step may be set to a value derived from a value of a bit thatwas determined in an immediately preceding decision.

The timeout signal may be generated based on a dynamically and/oradaptively determined threshold time. The determined threshold time maybe determined so that it guarantees that a magnitude of a differencebetween an analog input voltage to the asynchronous SAR ADC and ananalog output DAC voltage is within an overlapping range of voltagescorresponding to the one or more overlapping redundant bits. For eachcomparison step, an analog input voltage to the asynchronous SAR ADC maybe compared to an analog output digital-to-analog converter voltage. Acorresponding comparing time for each comparison step may be mapped tothe difference between the analog input voltage to the asynchronous SARADC and the analog output DAC voltage. Results from the mapping may becalibrated based on process, temperature and/or the analog input voltageto the asynchronous SAR ADC variations. The results from the mapping andresults from the calibrating may be stored, for example, in alook-up-table. A particular value for a next bit may be inferred if thecorresponding comparing time for each comparison step exceeds aparticular time, which provides an indication that a magnitude of adifference between the analog input voltage to the asynchronous SAR ADCand the analog output DAC voltage is less than a fractional portion of acurrent step size of a DAC that generates the analog output DAC voltage.Exemplary fractional portions may comprise ½, ¼, ⅛, 1/16, 1/32 and soon.

FIG. 1 is a block diagram illustrating a successive approximationregister ADC, in connection with various exemplary aspects of theinvention. Referring to FIG. 1, there is shown a successiveapproximation register ADC 100 comprising a summer 104, a comparator106, a search and decode logic module 108 and a digital-to-analogconverter (DAC) 110.

The summer 104 may comprise suitable logic, circuitry and/or interfacesthat may be operable to sum the input analog voltage, V_(in), with theoutput analog voltage, V_(dac), generated from the DAC 110 in order todetermine a difference between the input analog voltage, V_(in) and theoutput analog voltage, V_(dac). The resulting output from the summer104, which is based on the difference, between the input analog voltage,V_(in) and the output analog voltage, V_(dac) may be communicated to thecomparator 106. The summer 104 may comprise an analog voltage summer.

The comparator 106 may comprise suitable logic, circuitry, interfacesand/or code that may be operable to generate an output signal or codebased on the output signal that is generated by the summer 104. In thisregard, the comparator 106 may be operable to generate an output signalor code that is representative of the difference, between the inputanalog voltage, V_(in) and the output analog voltage, V_(dac). Thecomparator 106 may comprise an analog voltage comparator.

The search and decode logic module 108 may comprise suitable logic,circuitry, interfaces and/or code that may be operable to perform abinary search of the bits and accordingly generate a resulting code wordbased on the output signal or code from the comparator 106. In thisregard, the search and decode logic module 108 may be operable to searchfor a code that produces an analog voltage, V_(dac), from the DAC 110,which best approximates the voltage V_(in). The search and decode logicmodule 108 may comprise a binary search and decode circuitry.

The DAC 110 may comprise suitable logic, circuitry, interfaces and/orcode that may be operable to generate the analog voltage, V_(dac), basedon the resulting code word generated by the search and decode logicmodule 108. The analog voltage, V_(dac), may be communicated to thesummer 104.

In operation, the summer 104 is operable to sum the input analogvoltage, V_(in), with the output analog voltage produced by a DAC 110and accordingly generate a signal or code that may be representative ofthe difference between the input analog voltage, V_(in) and the outputanalog voltage, V_(dac). The comparator 106 may be operable to generatean output signal based on the difference between the input analogvoltage, V_(in) and the output analog voltage, V_(dac). The search anddecode logic module 108 may be operable to perform a binary search ofthe bits and accordingly generate a resulting code word based on theoutput signal from the comparator 106. In this regard, the search anddecode logic module 108 may be operable to search for the code thatproduces a DAC voltage that may be closest to the input analog voltage,V_(in). During the searching, time is allocated to allow the outputanalog voltage, V_(DAC), of the DAC 110 to settle followed by timerequired to compare the output analog voltage, V_(dac), with the inputanalog voltage, V_(in). In general, the maximum settling time for a DACmay be dependent on the settling time of the most significant bit (MSB).

FIG. 2 is a block diagram illustrating operation of an asynchronoussuccessive approximation register ADC, in connection with variousexemplary aspects of the invention. Referring to FIG. 2, there is showna timeline 200 of the operation of an asynchronous successiveapproximation register ADC without redundancy. The time allocated forthe output analog voltage, V_(DAC), of the DAC 110 to settle may bereferenced as t_(DAC) _(_) _(n). The time period 201 illustrates anexemplary time period, t_(DS) _(_) _(n), during which the DAC 110settles. The time allocated for the comparator 106 to compare anddetermine the value output from the summer 104 may be referenced ast_(C) _(_) _(n). The output from the summer 104 is the differencebetween the input analog voltage, V_(in), and the output of the analogvoltage, V_(dac), of the DAC 110. The time period 202 illustrates anexemplary period, t_(M) _(_) _(n), during which the comparator 106compares the output from the summer 104, which is the difference betweenthe analog voltage, V_(dac), and the input analog voltage, V_(in). Atthe end of the time period, t_(M) _(_) _(n), the comparator 106 makes adecision.

The successive approximation register ADC 100, which is illustrated inFIG. 1, relies on an asynchronous approach, which allocates apredetermined amount of time, t_(DAC) _(_) _(n), for the analog voltage,V_(dac), of the DAC 110 to settle and a predetermined amount of time,t_(C) _(_) _(n), for the difference of output analog voltage, V_(dac),of the DAC 110 and the input analog voltage, V_(in), t_(C) _(_) _(n) tobe determined by the comparator 106. Additionally, the successiveapproximation register ADC 100, which utilizes N bits, typically requireN steps in order to quantize the input analog voltage, V_(in). Each ofthe N steps involves a time period given by the sum of t_(DAC) _(_) _(n)and t_(C) _(_) _(n). Furthermore, both t_(DAC) _(_) _(n) and t_(C) _(_)_(n) are set to accommodate various worst-case scenarios. For example,t_(DAC) _(_) _(n) and t_(C) _(_) _(n) are set in order to allow the DAC110 to settle to within one lowest significant bit (LSB) of the inputanalog voltage, V_(in), or a wrong decision will be made in the binarysearch algorithm. In another example, since the input to the comparator106 may be very small (close to zero), this may result in a“meta-stable” condition where the comparator 106 takes an inordinatelylong time to settle. If the time t_(M) _(_) _(n) exceeds the allocatedtime t_(C) _(_) _(n), then an erroneous decision may be made in thebinary search that is executed by the search and decode logic module108. In order to address incomplete DAC settling, the DAC 110 may beoperable to utilize overlapping DAC ranges, which are also referred toas redundancy. The redundancy maps a particular voltage to more than oneDAC code word or value by designating one or more bits to have aredundant weighting to cover overlapping ranges of voltages. This allowsthe worst-case DAC settling time to be reduced so that the DAC is forcedto settle to within the overlapping range. Additional information on themeta-stable condition and meta-stable detection may be found in U.S.application Ser. No. 13/945,579 Jul. 18, 2013, which is herebyincorporated herein by reference in its entirety.

FIG. 3 is a diagram that illustrates DAC redundancy and its effect onDAC settling time for bit n, t_(DS) _(_) _(n), in connection withvarious exemplary aspects of the invention. Referring to FIG. 3, thereis shown a graph 300 of a conventional 16-level DAC without redundancyand a graph 310 of a 16-level DAC with redundancy.

Referring to the graph 300 of the conventional 16-level DAC withoutredundancy, at the end of the time period, t_(DS) _(_) _(n), duringwhich the DAC 110 settles, the final value of the DAC is ½ the LSB.

Referring to the graph 310 of the conventional 16-level DAC 110 withredundancy, there is shown a redundancy of 2 overlapping codesintroduced by a redundant DAC weighting of 2 at bit b1. The overlappingrange of voltages allows the DAC 110 additional range (DAC in range 312)in which to settle.

FIG. 4 is a block diagram of an exemplary asynchronous successiveapproximation register ADC, in accordance with an exemplary embodimentof the invention. Referring to FIG. 4, there is shown an asynchronoussuccessive approximation register ADC 400 comprising a summer 404, acomparator 406, a search and decode logic module 408, adigital-to-analog converter (DAC) 410 and a timer module 412.

The summer 404 may comprise suitable logic, circuitry and/or interfacesthat may be operable to sum the input analog voltage, V_(in), with theoutput analog voltage, V_(dac), generated from the DAC 410 in order todetermine a difference between the input analog voltage, V_(in) and theoutput analog voltage, V_(dac). The resulting output from the summer404, which is based on the difference, between the input analog voltage,V_(in) and the output analog voltage, V_(dac) may be communicated to thecomparator 406. The summer 404 may comprise an analog voltage summer.

The comparator 406 may comprise suitable logic, circuitry, interfacesand/or code that may be operable to generate a comparator output voltagesignal or code, V_(comp), based on the output signal that is generatedby the summer 404. In this regard, the comparator 406 may be operable togenerate a comparator output voltage signal or code, V_(comp), that isrepresentative of the difference, between the input analog voltage,V_(in) and the output analog voltage, V_(dac). The comparator 406 maycomprise an analog voltage comparator. The comparator 406 may also beoperable to generate a comparator output valid signal, V_(cov), whichindicates the completion of each step. The comparator output validsignal, V_(cov), is operable to provide an indication that thecomparator 406 has made a decision. The comparator output valid signalis utilized to control and provide asynchronous timing within theasynchronous successive approximation register ADC 400. In this regard,the start of each step in the asynchronous successive approximationregister ADC 400 is triggered by the completion of the previous step.

The search and decode logic module 408 may comprise suitable logic,circuitry, interfaces and/or code that may be operable to perform abinary search of the bits and accordingly generate a resulting codeword, D_(out), based on the output signal or code from the comparator406. The operation of the search and decode logic module 408 may betriggered by the comparator output valid signal, V_(cov), when thecomparator output valid signal, V_(cov), is active. In this regard, thesearch and decode logic module 408 may be operable to search for a codethat produces an analog voltage, V_(dac), from the DAC 410, which bestapproximates the voltage V_(in). The search and decode logic module 408may comprise a binary search and decode circuitry. In some embodimentsof the invention, a timer elapsed or timeout signal is generated by thetimer module 412, this may preempt the comparator output valid signal,V_(cov). In instances when the timer elapsed or timeout signal preemptsthe comparator output valid signal, V_(cov), this may force a preemptivedecision.

The DAC 410 may comprise suitable logic, circuitry, interfaces and/orcode that may be operable to generate the analog voltage, V_(dac), basedon the resulting code word, D_(out), generated by the search and decodelogic module 408. The analog voltage, V_(dac), which is generated by theDAC 410 may be communicated to the summer 404.

The timer module 412 may comprise suitable logic, circuitry, interfacesand/or code that may be operable to generate a timer elapsed (TE) signalor a timeout, which may be operable to force the search and decode logicmodule 408 to trigger a preemptive decision. The timer module 412 may beoperable to measure and/or estimate the actual time of each comparatorstep, t_(M) _(_) _(n), in order to trigger the preemptive decision. Inaccordance with various embodiments of the invention, the triggering ofthe preemptive decision may result in one or more (including all) of theremaining code word bits being set to a particular value based on thetime the timer module 412 determines is required or has taken or thecomparator 406 to arrive at a decision. In some embodiments of theinvention, the timer module 412 may be operable to generate a timeoutsignal that may preempt the comparator output valid signal, V_(cov),which may be generated by the comparator 406. In instances when thetimer module 412 generates the timer elapsed or timeout signal, thetimer elapsed or timeout signal preempts the comparator output validsignal, V_(cov), and forces the preemptive decision.

In operation, the summer 404 is operable to sum the input analogvoltage, V_(in), with the output analog voltage produced by a DAC 410and accordingly generate a signal or code that may be representative ofthe difference between the input analog voltage, V_(in) and the outputanalog voltage, V_(dac). The comparator 406 may be operable to generatethe comparator output voltage signal or code, V_(comp), and thecomparator output valid signal, V_(cov), based on the difference betweenthe input analog voltage, V_(in) and the output analog voltage, V_(dac).The comparator 406 may be operable to generate the comparator outputvalid signal, V_(cov), in order to provide an indication that thecomparator 406 has made a decision and that the comparator outputvoltage signal or code, V_(comp), is a valid output of the comparator406. The generation of the comparator output voltage signal or code,V_(comp), also provides asynchronous timing within the asynchronoussuccessive approximation register ADC 400.

The search and decode logic module 408 may be triggered upon receipt ofthe comparator output valid signal, V_(cov), which is generated by thecomparator 406. The search and decode logic module 408 may be operableto generate a start signal indicator (SSI) that starts the timer module412. The search and decode logic module 408 may be operable to perform abinary search of the bits and accordingly generate a resulting code wordbased on the output signal from the comparator 406 based on thecomparator output voltage signal or code, V_(comp), and the comparatoroutput valid signal, V_(cov), which are generated by the comparator 406.In this regard, the search and decode logic module 408 may be operableto search for the code that produces a DAC voltage that may be closestto the input analog voltage, V_(in).

The timer module 412 may be operable to generate a timer elapsed signal,TE, after a certain time period has elapsed. The generated timer elapsedsignal, TE, may be communicated from the timer module 412 to the searchand decode logic module 408. Upon receipt of the generated timer elapsedsignal, TE, or timeout signal from the timer module 412, the search anddecode logic module 408 may be operable to trigger a pre-emptivedecision. The search and decode logic module 408 may be operable togenerate an output signal, Dout, which comprises a code word.

The timer module 412 may comprise, for example, a ring oscillatoroperating at a relatively precise frequency, whose state is decoded toindicate a periodic time. In an alternate embodiment of the invention,the timer module 412 may comprise a counter that may be driven by, forexample, a voltage controlled oscillator (VCO) and/or by one or more byone or more prescalers driven by the voltage controlled oscillator. Thevoltage controlled oscillator may be a dedicated VCO or may be one thatis utilized in a reference synthesizer or radio frequency (RF)synthesizer that may already be present on a chip (i.e. one used for theRF signal path or a serializer/deserializer (SERDES) interface), inwhich the asynchronous successive approximation register ADC 400 may beutilized.

As utilized herein, the asynchronous timing that is utilized by theasynchronous successive approximation register ADC 400 means that thetime allocated for each step of the binary search that is executed bythe search and decode logic module 408 is variable. In this regard, theDAC settling time, t_(DS) _(_) _(n), and the comparator decision time,time t_(M) _(_) _(n), are variable. In this regard, the DAC settlingtime, t_(DS) _(_) _(n), and the comparator decision time, time t_(M)_(_) _(n), may be dynamically and/or adaptively variable. The start ofeach step is triggered by completion of the previous step. Thecomparator 406 is operable to generate the comparator output validsignal, V_(cov), which provides an indication completion of the decisionfor each step executed by the comparator 406. Once the comparator outputvalid signal, V_(cov), is active or asserted, the search and decodelogic module 408 is triggered and captures the comparator output voltagesignal or code, V_(comp), which is generated by the comparator 406. Inthis regard, the decode and binary search logic module 408 as well asthe DAC code word are triggered when the comparator output valid signal,V_(cov), is active or asserted. In an alternate embodiment of theinvention, the timer module 412 may be operable to generate a timeoutsignal that preempts the comparator output valid signal, V_(cov), andforces a preemptive decision.

In accordance with various embodiments of the invention, the timermodule 412 may be operable to measure and/or estimate the time requiredfor each comparator step, namely, the comparator decision time, t_(M)_(_) _(n). Once the comparator decision time, t_(M) _(_) _(n) haselapsed, the timer module 412 may be operable to trigger a preemptivedecision where one or more remaining code word bits are set to aparticular value based on the time required for the comparator 406 toarrive at a decision.

With regard to the preemptive decision, the precise comparator decisiontime, t_(M) _(_) _(n), which may be required for the comparator 408 tomake a decision may be utilized to set a certain number of subsequentDAC codeword bits. In general, the longer the comparator step, thecloser the analog voltage, V_(dac), from the DAC 410, should be to theanalog input voltage, V_(in). In instances when the comparator step hastaken a sufficiently long time to indicate that |V_(in)−V_(dac)| is lessthan approximately half the size of the current DAC step size, the valueof the next bit, may be inferred depending on the sign of(V_(in)−V_(dac)). In instances when the comparator step has taken asufficiently long time to indicate that |V_(in)−V_(dac)| is less than aquarter of the size of the current DAC step size, the value of the nexttwo bits may be inferred depending on the sign of (V_(in)−V_(dac)). Inthis manner an arbitrary number of subsequent DAC code word bits may bedetermined. In instances when the comparator step has taken sufficientlylong time to indicate that |V_(in)−V_(dac)| is less than a predeterminedDAC bit step size (N_(ovlp)), the value of the DAC code word bits up tothat bit step size may be inferred. The timer module 412 and/or thesearch and decode logic module 408 may be operable to determine whethera comparator step has taken a sufficiently long time for a condition tooccur. In instances where redundancy is utilized in the DAC 410 byintroducing a bit b_(r) with redundant weighting, preemptive decisionsmay be made on bits b_(n) to b_(r+1), where b_(n) is the current bit inthe successive approximation, since the settling time which is set bythe time elapsed or timeout signal only permits certainty to within therange provided by the overlapping codes.

In accordance with an exemplary embodiment of the invention, apreemptive decision may be triggered in instances when the timer module412 determines that a decision time has exceeded a particular thresholdvalue. In this regard, once the timer module 410 determines that thethreshold value has been exceeded, the timer module 410 may force thepreemptive decision. The threshold value may be chosen in order toguarantee that |V_(in)−V_(dac)| is within a decision range which may beequal to the overlap range. The value of the threshold may be static(fixed), varied dynamically and/or varied adaptively.

FIG. 5 is a diagram illustrating exemplary operation of an asynchronoussuccessive approximation register ADC, in accordance with an exemplaryembodiment of the invention. Referring to FIG. 5, the reference numerals501, 502 and 503 illustrates a timeline for a sequence of the exemplarysteps.

The reference 501 represents the settling time for the DAC 410. At 501,step n is started and time t_(DS) _(_) _(n) is allocated for the DAC 410to settle within the least significant bit (LSB) of the input voltage,V_(in), to prevent any errors. The time t_(DS) _(_) _(n) represents thetime required for the DAC 410 to settle. The start of each step, n, n−1,and so on (MSB to LSB), is triggered by completion of the previous step.

The reference 502 represents the comparator decision step. During thecomparator decision step, the comparator 406 is started and the timermodule 412 is also started. In this regard, the comparator 406 isstarted when the comparator output valid signal, V_(cov), becomes activeor is asserted. The timer module 412 is started when the search anddecode logic 408 generates a start step indicator signal to the timermodule 412.

The reference 503 illustrates the finishing of step n. In the finishingof step n, the comparator 406 may indicate a valid decision based on thecomparator output valid signal, V_(cov), or the timer module maygenerate a timeout or time elapsed (TE) signal, which indicates apreemptive decision. The comparator 406 is operable to generate thecomparator output valid signal, V_(cov), in order to indicate thecompletion of each step. In instances where the timer module 412generates a timeout or time elapsed (TE) signal to force a preemptivedecision, one or more bits in the DAC codeword bits may be set topreemptive values. For example, bits b_(n) through b_(r+i), may be setto preemptive values, where b_(n) is the current bit in the successiveapproximation and b_(r) is the bit with redundant weighting.

FIG. 6 is a diagram illustrating an exemplary preemptive decision withredundancy for an asynchronous successive approximation register ADC, inaccordance with an exemplary embodiment of the invention. Referring toFIG. 6, there is shown a timeout signal, V_(timeout), a comparatoroutput valid signal, V_(cov), a comparator output voltage signal,V_(comp), an analog output DAC voltage, V_(dac), from and an inputanalog voltage V_(in).

The decode and binary search logic module 408 is triggered when thecomparator output valid signal, V_(cov), is asserted or becomes active.

The timeout signal, V_(timeout), may be generated and output by thetiming module 412. The timeout signal, V_(timeout), may be substantiallysimilar to the TE or timeout signal generated by the timer module 412.

The comparator output valid signal, V_(cov), may be generated and outputby the comparator 406 and is utilized to provide an indication that theoutput of the comparator is valid.

The comparator output voltage signal, V_(comp), may be generated andoutput by the comparator 406.

The analog output DAC voltage, V_(dac), may be generated and output bythe DAC 410.

Referring to FIG. 5, the comparator 406 generates the comparator outputvoltage signal, V_(comp), and the comparator output valid signal,V_(cov), becomes active shortly thereafter to indicate that thecomparator output voltage signal, V_(comp), is valid. The analog outputDAC voltage, V_(dac), represents the DAC code word, 10 . . . 0, forexample. The DAC code word with the decision for the current bit is heldat 601. After a certain time has elapsed, and no decision was made, thetiming module 412 forces a preemptive decision and the timeout signal,V_(timeout), is asserted or becomes active. The value of the DACcodeword with the current decision is held. Since the prior decision wasa 0 (0?? . . . ?) and the current decision is a 0 (00? . . . ?), at 602,the remaining bits for the current decision may be set to the inverse ofthe current decision. Since the current decision is 0, the remainingbits in the DAC code word up to, but not including the redundant bit,are set at 1, which is the inverse of the current decision. The DAC codeword now becomes 0011 . . . 1??, where the last overlapping codes arerepresented by ??. The successive approximation algorithm resumes at bitbr and computes the final DAC code word as 0011 . . . 101.

In accordance with an embodiment of the invention, the time reported bythe timer module 412 may be mapped along with the voltage differencethrough computation or by utilizing a look-up table (LUT). Therelationship between the comparator decision time and the input voltagedifference |V_(in)−V_(DAC)| may be modeled theoretically or empiricallyand the resulting information may also be incorporated in the look-uptable. For example, the relationship between the comparator decisiontime and the input voltage difference may take into considerationprocess, voltage and/or temperature (PVT) variations. In this regard,the LUT may be calibrated based on the corresponding process,temperature and supply voltage data.

Since large voltage differences may be readily generated and typicallycorrespond to very short comparator decision times, the generatedvoltage differences may be applied to the comparator (reusing the DAC)and the corresponding comparator decision time may be measured. Thisinformation may be stored in the look-up table. In accordance with anembodiment of the invention, background calibration of the comparatordecision time may be done statically, dynamically and/or actively andthe corresponding comparator decision times may be stored in the look-uptable.

FIG. 7 is a flow chart illustrating an exemplary preemptive decisionwithout timeout, in accordance with an exemplary embodiment of theinvention. Referring to FIG. 7, there is shown a flow chart 700comprising exemplary steps 702 through 708. In step 702, the search anddecode logic (SDL) module waits for the comparator output valid signal,V_(cov), to become active before it starts the next step. In 704, thesearch and decode logic module starts the next step when the comparatoroutput valid signal, V_(cov), becomes active, sends a start stepindicator (SSI) signal to the timer module, and utilizes the comparatoroutput voltage signal, V_(comp), to make the current decision. In step706, the search and decode logic module sets the remaining bits in theDAC code word to the inverse of the current decision, up to the firstbit that corresponds to a step size that is smaller than the decisionrange. In step 708, the search and decode logic module continues withthe binary search process for the remaining bits that are in thedecision range.

FIG. 8 is a flow chart illustrating an exemplary preemptive decisionwith timeout, in accordance with an exemplary embodiment of theinvention. Referring to FIG. 8, there is shown a flow chart 800comprising exemplary steps 802 through 810. In step 802, the search anddecode logic (SDL) module waits for the comparator output valid signal,V_(cov), to become active before it starts the next step. In step 804,the search and decode logic module starts the next step when thecomparator output valid signal, V_(cov), becomes active, sends a startstep indicator (SSI) signal to the timer module and utilizes thecomparator output voltage signal, V_(comp), to make the decision for thecurrent step. In step 806, the timer module forces a preemptive decisionbased on the actual time the comparator takes to make a decision for thecurrent step. In step 808, since a timeout occurs, the SDL module setsone or more bits in the DAC codeword to certain preemptive values. Instep 810, the SDL module continues to the next step.

In accordance with various exemplary embodiments of the invention, anasynchronous successive approximation register analog-to-digitalconverter 400, which utilizes one or more overlapping redundant bits ineach digital-to-analog converter (DAC) code word, is operable togenerate an indication signal, V_(cov), which indicates completion ofeach comparison step and indicates that an output decision for the eachcomparison step is valid. A timer such as the timer module 412 may beinitiated based on the generated indication signal, V_(cov). A timeoutsignal, V_(timeout), may be generated that preempts the indicationsignal, V_(cov), and forces a preemptive decision.

The preemptive decision may be operable to set one or more remainingbits up to, but not including, the one or more overlapping redundantbits in a corresponding digital-to-analog converter code word for acurrent comparison step to a particular value. In an exemplaryembodiment of the invention, the one or more remaining bits up to, butnot including, the one or more overlapping redundant bits in thecorresponding digital-to-analog converter code word for the currentcomparison step may be set to a value derived from a value of a bit thatwas determined in an immediately preceding decision.

The timeout signal, V_(timeout), may be generated based on a dynamicallyand/or adaptively determined threshold time. The determined thresholdtime may be determined so that it guarantees that a magnitude of adifference between an analog input voltage to the asynchronous SAR ADC,V_(in), and an analog output DAC voltage, V_(dac), is within anoverlapping range of voltages corresponding to the one or moreoverlapping redundant bits. For each comparison step, an analog inputvoltage to the asynchronous SAR ADC, V_(in), may be compared to ananalog output digital-to-analog converter voltage, V_(dac). Acorresponding comparing time for the each comparison step may be mappedto the difference between the analog input voltage to the asynchronousSAR ADC, V_(in), and the analog output DAC voltage, V_(dac). Resultsfrom the mapping may be calibrated based on process, temperature and/orthe analog input voltage to the asynchronous SAR ADC, V_(in),variations. The results from the mapping and results from thecalibrating may be stored, for example, in a look-up-table. A particularvalue for a next bit may be inferred in instances where thecorresponding comparing time for each comparison step exceeds aparticular time, which provides an indication that a magnitude of adifference between the analog input voltage to the asynchronous SAR ADC,V_(in), and the analog output DAC voltage, V_(dac), is less than afractional portion of a current step size of a DAC that generates theanalog output DAC voltage. Exemplary fractional portions may comprise ½,¼, ⅛, 1/16, 1/32 and so on.

As utilized herein the terms “circuits” and “circuitry” refer tophysical electronic components (i.e. hardware) and any software and/orfirmware (“code”) which may configure the hardware, be executed by thehardware, and or otherwise be associated with the hardware. As usedherein, for example, a particular processor and memory may comprise afirst “circuit” when executing a first one or more line(s) of code andmay comprise a second “circuit” when executing a second one or morelines of code. As utilized herein, “and/or” means any one or more of theitems in the list joined by “and/or”. As an example, “x and/or y” meansany element of the three-element set {(x), (y), (x, y)}. As anotherexample, “x, y, and/or z” means any element of the seven-element set{(x), (y), (z), (x, y), (x, z), (y, z), (x, y, z)}. As utilized herein,the term “exemplary” means serving as a non-limiting example, instance,or illustration. As utilized herein, the terms “e.g.,” and “for example”set off lists of one or more non-limiting examples, instances, orillustrations. As utilized herein, circuitry is “operable” to perform afunction whenever the circuitry comprises the necessary hardware andcode (if any is necessary) to perform the function, regardless ofwhether performance of the function is disabled, or not enabled, by someuser-configurable setting.

Throughout this disclosure, the use of the terms dynamically and/oradaptively with respect to an operation means that, for example,parameters for, configurations for and/or execution of the operation maybe configured or reconfigured during run-time (e.g., in, or near,real-time) based on newly received or updated information or data. Forexample, an operation within a transmitter and/or a receiver may beconfigured or reconfigured based on, for example, current, recentlyreceived and/or updated signals, information and/or data.

Other embodiments of the invention may provide a computer readabledevice and/or a non-transitory computer readable medium, and/or amachine readable device and/or a non-transitory machine readable medium,having stored thereon, a machine code and/or a computer program havingat least one code section executable by a machine and/or a computer,thereby causing the machine and/or computer to perform the steps asdescribed herein for asynchronous successive approximation register(SAR) analog-to-digital converters.

Accordingly, the present invention may be realized in hardware,software, or a combination of hardware and software. The presentinvention may be realized in a centralized fashion in at least onecomputer system, or in a distributed fashion where different elementsare spread across several interconnected computer systems. Any kind ofcomputer system or other apparatus adapted for carrying out the methodsdescribed herein is suited. A typical combination of hardware andsoftware may be a general-purpose computer system with a computerprogram that, when being loaded and executed, controls the computersystem such that it carries out the methods described herein.

The present invention may also be embedded in a computer programproduct, which comprises all the features enabling the implementation ofthe methods described herein, and which when loaded in a computer systemis able to carry out these methods. Computer program in the presentcontext means any expression, in any language, code or notation, of aset of instructions intended to cause a system having an informationprocessing capability to perform a particular function either directlyor after either or both of the following: a) conversion to anotherlanguage, code or notation; b) reproduction in a different materialform.

While the present invention has been described with reference to certainembodiments, it will be understood by those skilled in the art thatvarious changes may be made and equivalents may be substituted withoutdeparting from the scope of the present invention. In addition, manymodifications may be made to adapt a particular situation or material tothe teachings of the present invention without departing from its scope.Therefore, it is intended that the present invention not be limited tothe particular embodiment disclosed, but that the present invention willinclude all embodiments falling within the scope of the appended claims.

What is claimed is: 1-20. (canceled)
 21. An analog-to-digital converter(ADC), comprising: a comparator comprising an analog input, a referenceinput, a preemption input, a codeword output, and a validation output, acodeword on the codeword output of the comparator comprising one or moreoverlapping redundant bits, wherein if a preemption is indicated at thepreemption input, the validation output is set to indicate thepreemption and one or more bits of the codeword are set to a particularvalue, and wherein if a preemption is not indicated at the preemptioninput, the validation output is set to indicate a valid decision and oneor more bits of the codeword are set according to a comparison betweenthe analog input and the reference input; a digital-to-analog converter(DAC) comprising a codeword input and a reference output, wherein thereference output of the DAC is operably coupled to the reference inputof the comparator and the codeword input of the DAC is operably coupledto the codeword output of the comparator; and a timer operable to set apreemption output in time according to a validation input, wherein thevalidation input of the timer is operably coupled to the validationoutput of the comparator and the preemption output of the timer isoperably coupled to the preemption input of the comparator.
 22. The ADCaccording to claim 21, wherein the comparator is operable to determineone or more bits of the codeword according to one or more comparisons.23. The ADC according to claim 21, wherein the comparator is operable toset one or more least significant bits of the codeword up to, but notincluding, the one or more overlapping redundant bits in the codewordfor a current comparison, the one or more least significant bits beingset to a value derived from a value of a bit that was determined in animmediately preceding comparison.
 24. The ADC according to claim 21,wherein the timer is operable to indicate a preemption at the preemptionoutput based on a adaptively-determined threshold time.
 25. The ADCaccording to claim 24, wherein the adaptively-determined threshold timeis determined according to a time required for a difference between theanalog input and the reference input to be less that a predeterminedthreshold.
 26. The ADC according to claim 25, wherein the predeterminedthreshold is based on the one or more overlapping redundant bits. 27.The ADC according to claim 25, wherein the comparator is operable to mapa comparing time for each comparison to a difference between the analoginput and the reference input.
 28. The ADC according to claim 27,wherein the comparator is operable to calibrate results from the mappingbased on process, temperature and/or variations in the analog input. 29.The ADC according to claim 28, wherein the comparator is operable tostore the results from the mapping and results from the calibrating. 30.The ADC according to claim 27, wherein the DAC is operable to set aparticular value for a next bit if the comparing time of each comparisonexceeds a particular time.
 31. A method, comprising: comparing an analogsignal to a reference signal to determine one or more bits of acodeword, the codeword on the codeword comprising one or moreoverlapping redundant bits; if a preemption is indicated, a validationsignal is set to indicate the preemption and one or more bits of thecodeword are set to a particular value; if a preemption is notindicated, the validation signal is set to indicate a valid decision andone or more bits of the codeword are set according to the comparisonbetween the analog input and the reference input; generating thereference signal according to the codeword using a digital-to-analogconverter (DAC); and setting the preemption according to a timer and thevalidation signal.
 32. The method according to claim 31, wherein themethod is performed iteratively.
 33. The method according to claim 32,wherein the method comprises determining one or more least significantbits of the codeword up to, but not including, the one or moreoverlapping redundant bits in the codeword for a current iteration, theone or more least significant bits being set to a value derived from avalue of a bit that was determined in an immediately precedingiteration.
 34. The method according to claim 31, wherein the methodcomprises indicating a preemption based on a adaptively-determinedthreshold time.
 35. The method according to claim 34, wherein the methodcomprises determining the adaptively-determined threshold time accordingto a time required for a difference between the analog input and thereference input to be less that a predetermined threshold.
 36. Themethod according to claim 35, wherein the predetermined threshold isbased on the one or more overlapping redundant bits.
 37. The methodaccording to claim 35, wherein the method comprises mapping a comparingtime for each comparison to a difference between the analog input andthe reference input.
 38. The method according to claim 37, wherein themethod comprises calibrating results from the mapping based on process,temperature and/or variations in the analog input.
 39. The methodaccording to claim 38, wherein the method comprises storing the resultsfrom the mapping and results from the calibrating.
 40. The methodaccording to claim 37, wherein the method comprises setting a particularvalue for a next bit if the comparing time of each comparison exceeds aparticular time.